Researchers have constructed a new synthetic bacterium that detects Pseudomonas aeruginosa, a common microbe and a leading cause of hospital-acquired infections, and explodes, releasing antimicrobials that kill the invaders. The results, published today (August 16) in Molecular Systems Biology, suggest that the engineered bacteria might eventually be used to prevent or treat infection with P. aeruginosa in humans.

"The paper sets up innovative use of synthetic biology for engineering microbes to carry out functions that they normally wouldn't do"—namely, kill other bacteria, said William Bentley, chair of the Fischell Department of Bioengineering at the University of Maryland in College Park, who was not involved with the research.

P. aeruginosa is an infectious bacterium that colonizes human respiratory and gastrointestinal tracts and rapidly develops resistance to antibiotics. The bacteria cause about 10 percent of all hospital-acquired infections, and are especially common in immunocompromised patients. Infections are treated using a combination of antibiotics, but this approach also eliminates symbiotic bacteria, which may make the body more susceptible to colonization by harmful bacteria. Phage therapy, the use of specific viruses to destroy the bacteria, is another potential treatment, although it is complicated by the fact that hosts may eventually make antibodies against the virus, preventing it from killing the bacteria.

So Chueh Loo Poh and Matthew Wook Chang at Nanyang Technological University in Singapore decided to devise a new way to fight the microbe—with another bacterium. Specifically, their teams engineered Escherichia coli bacteria to detect molecules involved in P. aeruginosa quorum sensing, called acyl homoserine lactones. The release of these molecules by P. aeruginosa triggered the engineered E. coli to produce pyocin S5, a protein antibiotic that is not normally produced by E. coli and that has been shown to kill P. aeruginosa. In the process of releasing pyocin S5, the engineered bacteria burst, killing themselves.

"That was a very clever aspect of this work, to design a delivery system that would grow and replicate until it found its target, and then would kill itself and its target at the same time," said microbiologist Beth Lazazzera at the University of California, Los Angeles, who was not involved with the study.

Poh and Chang's teams tested the effectiveness of the engineered bacteria against P. aeruginosa cultured either free floating in media or in a biofilm assay, and found that the engineered bacteria inhibited growth of P. aeruginosa and prevented biofilm formation.

The fact that the scientists engineered the killer bacteria to sense P. aeruginosa’s unique quorum-sensing molecules—which allow bacteria to monitor population cell density, and in turn regulate development, virulence and other processes—suggests a similar strategy may prove successful for other quorum-sensing microbes, such as Vibrio cholera and Helicobacter pylori. John March's lab at Cornell University in Ithaca, New York, for example, has already created engineered bacteria that target V. cholerae through its quorum sensing molecules, and the group has shown that the approach can protect against cholera in a mouse model.

"We can easily develop another type of engineered bacteria to target other infectious pathogens," said Chang, a chemical and biomolecular engineer. First, however, the group plans to test their new system in mice that have been infected with P. aeruginosa, and they expect to have results within the next few years.

If the engineered E. coli prove successful in mouse models and other future studies, Poh and Chang envision that they could be administered in a probiotic drink that immunocompromised patients could take to prevent infection. Because the bacteria are not known to be harmful, they could live in the intestinal tract, where E. coli are naturally plentiful, until they encountered infection. Furthermore, pyocin S5 does not kill E. coli or other microbes, meaning the treatment wouldn’t affect the body’s symbiotic inhabitants, Chang and Poh said.

But some questions remain as the group moves into in vivo experiments. For example, it is unclear whether the engineered bacteria will be able to locate P. aeruginosa within infected organisms. "It's not obvious that it would necessarily find them," Bentley noted. "There's nothing that drives the E. coli to the Pseudomonas." Other types of quorum sensing molecules that act as homing mechanisms could be added to Chang and Poh's system to ensure that the engineered bacteria more easily target the infection, Bentley added.